Method for producing a monocrystalline body from a magnetic shape memory alloy

ABSTRACT

A method for producing an MSM actuator element, having a crystal orientation along a first crystal axis, from a monocrystalline MSM body by introducing a molten alloying material into a molding shell and subsequently solidifying the alloying material, comprising the following steps: providing a molding shell which comprises a nucleation region ( 24 ), a selector region ( 26 ) and a crystal region ( 28 ) and is oriented along a longitudinal axis ( 22 ) at least in some sections, introducing the molten MSM alloying material, in particular NiMnGa-based alloying material, into the molding shell without providing a separate nucleation crystal, compacting the MSM alloying material by generating a solidification front moving from the nucleation region across the selector region into the crystal region along a solidification path.

BACKGROUND OF THE INVENTION

The present invention relates to method for producing a monocrystallineMSM body for the production of an MSM actuator and such amonocrystalline MSM body, as is produced by the method.

MSM actuators (also designated “MSM-actuators”) are generally known fromthe prior art and utilize the effect that under the influence of amagnetic field, so-called magnetic shape memory materials (MSM=MagneticShape Memory) carry out an expansion movement which—typically lyingalong the expansion direction in the single-figure percent rangerelative to a length of a respective body—can be the basis for a driveand in this respect can be for instance an alternative to knownactuators realized by means of permanent magnets and/or electromagnets.

In addition to the alloy which is used (typically an alloy on the basisof NiMnGa), the crystal orientation in which the MSM element is presentis critical for the effectiveness of such an MSM actuator orrespectively MSM actuator element: Methods to be assumed as being knownfrom the prior art for the production of monocrystalline MSM materialhave the characteristic that a crystal orientation resulting by theintroduction of a molten alloying material into a molding shell andsubsequent cooling or respectively solidification of the alloyingmaterial is stochastic, with the result that an alignment of the crystalaxes is not predeterminable and must then be developed by subsequentmanufacturing steps of the MSM body. FIG. 5 shows such an arrangement ofthe prior art: An MSM monocrystal 10 which has been solidified andelongated in the previously described manner has a geometriclongitudinal axis 12 determined by the molding shell. Largely in astochastic manner during the solidification of the material, however, acrystal orientation has formed in the monocrystal 10, which is describedby way of example by a first crystal axis 14 and a second crystal axis16 orthogonal thereto (wherein the third axis is then automaticallyfixed orthogonally to them both). This then leads to an MSM element 18(after prior determining of the crystal axes by measuring) being able tobe cut out from the finished monocrystal, which has a maximum dimensiondelimited by the geometric relationships which are shown (with a largeamount of waste material, accordingly). Consequently, this leads to thefact that with prevalent longitudinal dimensions of monocrystalline MSMelements in the range between 10 mm and 30 mm and with desiredcross-sections of typically between 5 mm² and 30 mm² correspondinglylarge monocrystals 10 (FIG. 5) must be produced, in order to also beable to manufacture the desired minimum dimensions for the MSM elementfor the case of unfavourable crystal orientations. It is obvious thatthis procedure, which is to be assumed as being known, is inefficient inmany respects; on the one hand, waste material occurs to a considerableextent through the necessary cutting processes (typically carried out bywire eroding), on the other hand in each case a measuring of theproduced monocrystal is necessary with regard to determining the crystalorientation (typical procedure by X-ray diffractometry), in order tocreate the prerequisite at all for the subsequent cutting.

It can also be seen from observing by way of example the geometricrelationships of FIG. 5 that the maximum achievable dimensions (e.g. alongitudinal extent of an MSM element which is to be produced) arelimited.

It is known from the prior art that so-called nucleation crystals (seedcrystals) can influence a crystal orientation in a monocrystalproduction process. For this purpose substantially a suitablemonocrystal, oriented in the desired manner, is incorporated into theprocess at the start of the process, on which the crystal which is to beproduced ideally nucleates in a monocrystalline manner. However, thisprocedure is also problematic in many respects; not only are suitablenucleation crystals costly and difficult to handle in particular forindustrial manufacturing processes outside a laboratory environment,also such nucleation crystals require a very precise process management,in order to achieve the correct nucleation behaviour (with furtherpossible disadvantages to the MSM effect of a produced MSM element ifthe nucleation crystal has material which is foreign to the alloy).Therefore, in addition to the obvious need for an increase in efficiencyaccording to the problem described above, the need also exists forprocedural simplification, with the aim of enabling processes which aresimple to operate and are potentially large-scale for the production ofmonocrystal MSM bodies.

With regard to the further prior art, reference is made to the followingdocuments:

1. M. ZHU et al.: “Preparation of single crystal CuAlNiBe SMA and itsperformances”, JOURNAL OF ALLOYS AND COMPOUNDS, Vol. 478, No. 1-2, 10Jun. 2009, page 404-410;

2. H. ESAKA et al.: “Analysis of single crystal casting process takinginto account the shape of pigtail”, MATERIALS SCIENCE AND ENGINEERING A,Vol. 413-414, 15 Dec. 2005, pages 151, 155;

3. GB 2 330 099 A;

4. C. Li et al.: “Preparation of Single Crystal of TiNi Alloy and itsShape Memory Performance”, PROC. OF SPIE. Vol. 7493, 2009, pages7493L1-74931L8;

5. K. ROLFS et al.: “Double twinning in NiMnGaCo”, ACTA MATERIALIA, Vol.58, No. 7, 1 Apr. 2010, pages 2646-2651;

6. U.S. Pat. No. 5,062,469 A;

7. M. LANDA et al.: “Ultrasonic characterization of Cu—Al—Ni singlecrystals lattice stability in the vicinity of the phase transition”,ULTRASONICS, Vol. 42, No. 1-9, 1 Apr. 2004, pages 519-526; 8. F. XIONGet al.: “Fracture mechanism of a Ni—Mn—Ga ferromagnetic shape memoryalloy single crystal” JOURNAL OF MAGNETISM and MAGNETIC MATERIALS, Vol.285, No. 3, 1 Jan. 2005, pages 410-416.

It is therefore an object of the present invention to provide a methodfor the production of a monocrystal MSM body and a correspondingmonocrystalline MSM body, which are improved with regard to utilizationof material and efficiency of the associated monocrystal material, inparticular to reduce waste of the monocrystal material for theproduction of one or more MSM elements from the monocrystalline MSM bodyand in addition to make the necessity of a nucleation crystalunnecessary.

SUMMARY OF THE INVENTION

The object is achieved by providing a method to provide a monocrystalMSM body, (preferably for use as an actuator or respectively actuatorelement), which proceeds from the fact that the divided body (anddivided further into individual actuator elements) resulting from themethod of the present invention is further treated with typical (andotherwise known) heat treatment steps and/or magnetomechanical trainingsteps, in order to achieve or respectively optimize the magnetic shapememory behaviour. In accordance with a further development, it is alsoin particular included by the invention to subject individual or theplurality of MSM actuator elements to a heat treatment after thedivision, in order to stimulate the magnetic shape memory behaviour;alternatively, this heat treatment can also take place on the compactedMSM alloying material before the division into the plurality of MSMactuator elements. Provision is also made within preferred furtherdevelopments of the invention to move the separated (distributed) MSMactuator elements in the manner of a training in a targeted andpredetermined manner in order to stimulate the shape memory behaviour.Provision is made here in particular (and otherwise also assumed asknown), that a separated element is moved in a targeted manner in theprovided expansion direction, for instance by the application of tensileand/or pressure forces, in order to thus carry out the training with theaid of such mechanical strokes.

In an advantageous manner according to the invention, the production ofthe monocrystal MSM body—preferably on the basis of a NiMnGaX alloyingmaterial, wherein X has optionally one or more elements of the group Co,Fe and Cu—without the necessity to provide a (separated) nucleationcrystal, rather solely from the introduction of the molten MSM alloyingmaterial into the especially configured molding shell according to theinvention. More precisely, the latter has a longitudinal axis and isdeflected in the region of the selector region from this longitudinalaxis, according to the invention by a deflection which exceeds themaximum cross-sectional width in the selector region. Thereby, provisionis made within the scope of the invention to realize a longitudinalsectional geometry of the solidification path deflecting according tothe invention by the formation of the selector region so that thisdeflection is greater in the cross-sectional direction than a maximumcross-section width in the selector region, in other words, the regionof the maximum deflection lies outside a projection of the cross-sectionin the crystal region to the entry of the selector region along thelongitudinal axis.

According to a further development, this deflection has in longitudinalsection the form of at least one spike, alternatively a spiral, a helixor another angle configuration.

Through this advantageous provision, the crystal structure of thesolidifying or respectively then solidified MSM material then undergoesin a manner according to the invention a crystal orientation whichorients itself on the longitudinal axis, more precisely runs along thedirection of the longitudinal axis of the molding shell (or respectivelydeviates therefrom by an angle deviation which according to theinvention is <10°, according to a further development is advantageously<6°, again according to a further development and advantageously is lessthan 3°).

It is thereby then advantageously achieved through the present inventionthat (with this then negligible orientation error in the practicalrealization) a monocrystal is produced, the crystal orientation of whichno longer occurs stochastically, but rather is marked by the mechanicalalignment of the molding shell along the longitudinal axis (orrespectively of the course section formed in a deflected manneraccording to the invention for the solidification in the selectorregion). The advantageous consequence resulting therefrom for seriesproduction is evident: Not only is the waste which is necessary in afurther treatment or respectively in the dividing of the solidifiedmaterial into a plurality of MSM elements drastically reduced, alsothrough the procedure according to the invention at least the crystalorientation is fixed with regard to the longitudinal axis through theprocedure according to the invention, in other words, before a possiblefurther treatment of the monocrystal for the realization of the MSMelement(s), a laborious step of orientation measurement (for instance bymeans of X-ray diffractometry) would be superfluous.

If then, as provided advantageously and according to a furtherdevelopment the molding shell (in particular in the selector orrespectively crystal region) is configured so as to be rectangular incross-section, in addition the crystal orientation of the solidifying orrespectively solidified MSM material can be influenced along a secondcrystal axis running orthogonally to the first crystal axis (and henceautomatically to the third orthogonal axis), so that as a result in thisway then also the complete three-dimensional crystal orientation of aresulting crystal is determined in the space (again without thenecessity of measuring).

Within the practical realization of the invention, it is particularlyfavourable and preferred to provide the longitudinal axis in verticaldirection, so as to provide it approximately perpendicularly to an(otherwise known) cold plate as cooling device in or at the nucleationregion of the molding shell. If the molding shell is then (in anotherwise known manner) moved from a warmth or respectively heatenvironment, opposed to the longitudinal axis, with a drawing speed,alloying material which is introduced into this molding shell in liquidstate solidifies owing to the temperature gradient then in an upwarddirection along a solidification path, which is able to be describedthrough the longitudinal axis and, deviating therefrom, is deflectedaccording to the invention in the selector region. According to afurther development advantageously the solidification—or respectivelycooling behaviour of the molding shell is arranged here so that incross-section (radially) no significant temperature gradient is presentfrom the interior outwards in the melt adjacent to the solidificationfront, and a temperature gradient of the melt close to thesolidification front is set at values of between 0.3 K/mm and 20 K/mm,wherein a particularly preferred range of values for producing thedesired crystal orientation lies in the range between 1 K/mm and 15K/mm. Additionally or alternatively, it is favourable according to afurther development to arrange the cooling rate, described by the speedof movement of the solidification front along the solidification path(or respectively a drawing speed of the molding shell relative to thetemperature gradient), at a range of between 0.1 ram/min and 10 mm/min,wherein a particularly preferred range lies between 0.3 mm/min and 5mm/min.

In this way a monocrystalline solidification behaviour is thenadvantageously achieved, which forms the first crystal axis of thecrystal structure at least along the longitudinal axis (or respectivelyshows between these axes a maximum angular deviation of less than 10°,typically less than 6° or even less than 3°). For the case whereaccording to a further development advantageously also the cross-sectionof selector region and/or crystal region (i.e. the plane perpendicularto the longitudinal axis) is configured so as to be rectangular, morepreferably square), in addition an influencing (parameter) of theorthogonal second or respectively third crystal axis can be achieved inthe direction of the rectangular longitudinal edges in cross-section, sothat in the ideal case of an e.g. elongated and cross-sectionallyrectangular crystal region of the molding shell, this region determinesthe three-dimensional orientation of a monocrystal which is solidifiedtherein. According to a further development, it is particularlyadvantageous within the scope of the invention to carry out a division,following the solidification, (always) perpendicularly to thelongitudinal axis (Z axis), because indeed in this respect, with thepreviously described maximum deviations, the crystal orientation isalready fixed.

As a result, therefore the present invention not only enables a drasticreduction in manufacturing steps or respectively upstream testing steps(because ideally any measuring of the crystal orientation can bedispensed with), the invention also permits MSM elements to be producedwhich are optimized with regard to dimension from the restrictedinterior of a molding shell, because in particular already in thedescribed molding process by solidification along a solidificationdirection corresponding to the longitudinal axis of the molding shelland a crystal alignment effected therewith, a maximum length dimensionis able to be produced. It is then to be expected in particular that MSMelements can be produced efficiently and with small manufacturingexpenditure (and hence potentially on a large scale) as the basis forthe production of MSM actuators (also by further dividing, e.g. sawing),which reach length dimensions of more than 20 mm, in particular morethan 40 mm and/or permit a cross-sectional area of 15 mm² or more.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the invention will emergefrom the following description of preferred example embodiments and withthe aid of the figures; these show in:

FIG. 1 a geometric schematic diagram of a molding shell arrangement forcarrying out the method according to a first example embodiment of theinvention;

FIG. 2 an illustration analogous to FIG. 1, but with a differentgeometrical configuration in the form of a cross-sectionally rectangularcrystal region of the molding shell;

FIG. 3 a diagrammatic illustration of a cylindrical MSM monocrystal andof a crystal orientation drawn therein diagrammatically on realizationof the invention;

FIG. 4 an illustration analogous to FIG. 3, but with a monocrystal bodyin the shape of a rectangular block to illustrate the crystalorientation of an MSM actuator element (likewise in the shape of arectangular block) provided diagrammically therein and

FIG. 5 a diagrammatic illustration of an MSM monocrystal realizedaccording to a generic method from the prior art with crystal axesoriented therein stochastically, and with the limited cut possibilitiesresulting therefrom for an MSM actuator element.

DETAILED DESCRIPTION

FIG. 1 illustrates the principle by which the present invention can berealized according to a first example embodiment. A so-called moldingshell is shown for the production of monocrystalline bodies by theso-called Bridgman method, which, extending from a cold plate 20perpendicularly along a longitudinal axis (dot-and-dashed line 22),forms a nucleation region 24, subsequently a selector region 26 and acrystal region 28. Suitably molten alloying material is introduced intothe device through an upper opening 30, and the liquid alloying materialthen solidifies from below upwards (arrow direction 32) with theformation of a correspondingly upward moving solidification front, thespeed of movement of which is predetermined by a suitable temperatureinfluence.

FIG. 1 (as also the analogous FIG. 2) illustrate how according to theinvention the solidification path takes place not perpendicularly andlinearly along the longitudinal axis 22, but rather has a linear coursewhich is bent in longitudinal section from FIG. 1 or respectively FIG.2; more precisely, in the selector region 26 the molding shell isconfigured so that its interior channel which is effective for thesolidification (from the direction from below upwards) firstly isdeflected by an angle □ of approximately 40° and then has a further, butoppositely deflected section, until the channel at the upper end of theselector region is again in alignment cross-sectionally with thecross-section on the base side. In accordance with the invention,advantageously this deflection, which in the illustrated exampleembodiment at its maximum lateral deflection transcends over theprojection of the cross-section in the crystal region or respectively inthe base region adjacent to the cold plate 20, advantageously providesfor a longitudinal orientation of the crystal structures in verticaldirection, i.e. in the direction of the axis 22. In the solidifiedstate, this then leads in the region of the crystal region 28 to themonocrystal which is present there having an orientation which has atleast a first crystal axis orientated in the direction of thelongitudinal axis (wherein here according to the invention a maximumangle error of 10°, typically however of less than 6° or even less than3° can be achieved).

FIG. 2 shows a variant of the example embodiment of FIG. 1; here in thecrystal region 28′, the channel extending vertically along thelongitudinal axis 22 is square in cross-section, so that, in addition toa crystal axis orientation in vertical direction, additionally the twocrystal axes orthogonal thereto extend parallel to the edge courses ofthe crystal region. FIG. 3 or respectively 4 illustrate thesegeometrical relationships, in this respect in accordance with the formsof realization of FIG. 1 or respectively FIG. 2: FIG. 3 shows the resultof a monocrystal body solidified in a hollow cylindrical crystal region.The direction of the longitudinal axis (here: z-axis) correspondsapproximately to the alignment of the crystal longitudinal axis c withthe described small possible angle error. Owing to the cylinderstructure (i.e. circular shape in the x-y plane in FIG. 3), the twofurther axes, orthogonal to one another and to the vertical axis c, arestochastic in their alignment. In contrast, the further development ofFIG. 2 (geometry according to FIG. 4) offers the possibility, byprovision of the square cross-sectional contour (running here parallelto the x- or respectively y-direction), to develop the second (a) orrespectively third (b) crystal axis parallel accordingly, so that as aresult of the carrying out of the method described in FIG. 2 orrespectively FIG. 4 a monocrystal is achieved, which through its shapein the form of a rectangular block already to the greatest possibleextent also describes its actual crystalline orientation and in thisrespect is potentially not (or only minimally) in need of furthertreatment. Also, the result of the production method according to FIG. 1(FIG. 3) is already advantageous in so far as here with the crystal axis(c), running in the direction of the longitudinal extent (z) of themolding shell and of the blank which is solidified therein, a relevantalignment is fixed for instance for the expansion behaviour of an MSMbody, and also such a cylindrical body is then able to be used withoutfurther (or only with minimal) further treatment, if the precisealignment of the a- or respectively b-crystal axes is not concerned.

The execution of the method is described below with the aid of apractical example:

Primary alloying material is produced as so-called master alloy byinduction melting from the materials NiMnGa, in accordance withcomposition for an MSM alloy, by induction melting. A typical meltingtemperature is set at a range of between 50° and 400° above theliquefaction temperature of the respective alloy. Typically, the meltingtakes place under an Ar atmosphere between 100 mbar and 1200 mbar.

The liquid master alloy is poured into a ceramic molding shell which hasa geometry in accordance with FIG. 1. This molding shell is moved in theBridgman method relative to a temperature gradient from a hot zone intoa cold zone, so that the solidification front runs through the moldingshell from bottom to top. This speed of the movement of thesolidification front typically lies at 0.3 mm/min; the temperaturegradient in the melt close to the solidification front is set at a valueof typically 3 K/mm. After running through the selector region, which isadvantageously deflected according to the invention, the MSM materialsolidifies with a crystal axis aligned vertically, i.e. along thedirection of the longitudinal axis 22, so that after concludingsolidification and cooling, a cylinder can be removed from the crystalregion 28 as an MSM body of the geometry shown in FIG. 3. This nowoffers the possibility of immediately realizing an MSM actuator with amovement-(expansion) direction extending axially; alternatively, fromthis body, by determining a crystalline transverse axis, theprerequisite can be created so that with little waste and minimized losson the covering surface side, one or more MSM elements which arecross-sectionally rectangular or respectively in the shape of arectangular block can be created with a defined crystal orientation alsoin the transverse direction. For such a separation, in particular cutsperpendicular to the Z-axis present themselves, because indeed in thisrespect the orientation is already developed.

To stimulate or respectively realize a complete shape memoryfunctionality of actuators realized in the described manner, thematerial is heat-treated (either as a whole body before the separation,alternatively by heat treatment of the divided individual actuatorelements). It is also advantageous to train these elements afterdividing in their movement—or respectively expansion behaviour, whereinfor this purpose, typically over some strokes, in the providedexpansion—or respectively movement direction a movement is imprintedinto the material by corresponding input of tensile force orrespectively pressure force.

Whereas the arrangement described above and the operation thereof forrealizing the method according to the invention are to be understoodgenerically and in principle (and configured and adapted in a suitablemanner by the specialist in the art), it is in particular also withinthe scope of the present invention to provide in the manner of amulti-armed molding shell a plurality of solidification paths alongselector—and crystal regions which are respectively separated from oneanother but nevertheless adjacent.

The ranges of application of an MSM body which is produced by thepresent invention are potentially unlimited; it is advantageously to beexpected that the present invention nevertheless considerably simplifiesand configures more economically the large-scale production of suchbodies which are clearly defined with regard to the crystal geometry, sothat in future further fields of application are developed for MSMactuators.

1-12. (canceled)
 13. A method for producing an MSM actuator element,having a determined crystal orientation along a first crystal axis, froma monocrystal MSM body by introducing a molten alloying material into amolding shell and subsequent solidification of the alloying material,comprising the steps of: (a) providing a molding shell which comprises anucleation region, a selector region and a crystal region having a firstcrystal axis oriented in the direction of a longitudinal axis at leastin some sections; (b) introducing a molten MSM alloying material intothe molding shell without providing a separate nucleation crystal; (c)compacting the MSM alloying material by generating a solidificationfront moving from the nucleation region across the selector region intothe crystal region along a solidification path, wherein thesolidification path in the crystal region runs along the longitudinalaxis, forms a region which is deflected from the longitudinal axis inthe selector region, the maximum deflection of which, relative to thelongitudinal axis, is greater than a maximum cross-sectional width inthe selector region, wherein the longitudinal axis has an angulardeviation of less than 10° from the first crystal axis; and (d) dividingthe solidified MSM alloying material into a plurality of MSM actuatorelements by cuts perpendicularly to the longitudinal axis.
 14. Themethod according to claim 13, wherein the solidification path in theselector region forms a region which is deflected in a spike-like mannerwith two angled sections, the entry and exit side of which is aligned inalignment to the longitudinal axis.
 15. The method according to claim13, wherein the solidification path in the selector region forms ahelix-shaped or zigzag-shaped region.
 16. The method according to claim13, wherein the longitudinal axis is aligned vertically to a flatcooling device associated with the nucleation region.
 17. The methodaccording to claim 13, wherein the crystal region extending in anelongated manner along the longitudinal axis has an effectivecross-sectional area for the solidification front of >3 cm².
 18. Themethod according to claim 13, wherein the crystal region extending in anelongated manner along the longitudinal axis has an effectivecross-sectional area for the solidification front of >7 cm².
 19. Themethod according to claim 13, wherein the crystal region extending in anelongated manner along the longitudinal axis has an effectivecross-sectional area for the solidification front of >12 cm².
 20. Themethod according to claim 13, wherein the MSM alloying material forproducing the solidification front is cooled so that a temperaturegradient in the melt, occurring in the selector region, adjacent to thesolidification front, is between 0.3 K/mm and 20 K/mm.
 21. The methodaccording to claim 13, wherein the MSM alloying material for producingthe solidification front is cooled so that a temperature gradient in themelt, occurring in the selector region, adjacent to the solidificationfront, is between 1 K/mm and 15 K/mm.
 22. The method according to claim13, wherein the MSM alloying material for producing the solidificationfront is treated by bringing about a relative speed between the moldingshell and the temperature gradient, so that the solidification front inthe selector region moves at a speed of between 0.1 mm/min and 50 mm/minalong the solidification path.
 23. The method according to claim 13,wherein the MSM alloying material for producing the solidification frontis treated by bringing about a relative speed between the molding shelland the temperature gradient, so that the solidification front in theselector region moves at a speed of between 0.3 mm/min and 5 mm/minalong the solidification path.
 24. The method according to claim 13,wherein the solidification front moves along the solidification paththrough an at least partially cross-sectionally rectangular crystalregion.
 25. The method according to claim 24, wherein across-sectionally rectangular inner contour of the crystal regiondetermines a crystal orientation of the MSM alloying material, which issolidified in a monocrystalline manner, in at least a second crystalaxis orthogonal to the first crystal axis.
 26. The method according toclaim 13, wherein the alloying material has Ni, Mn, Ga and at least Coin the composition Ni_(a)Mn_(b)Ga_(c)Co_(d)Fe_(e)Cu_(f), wherein a, b,c, d, e and f are indicated in atom-% and fulfill the conditions44≦a≦51;19≦b≦30;18≦c≦24;0.1≦d≦15;0≦e≦14.9;0≦f≦14.9;d+e+f≦15;a+b+c+d+e+f=100.
 27. The method according to claim 13, wherein thecompacting of the MSM alloying material takes place along a plurality ofsolidification paths which are adjacent to one another and separatedfrom one another.
 28. The method according to claim 13, includingdividing of the MSM alloying material, which is solidified in thecrystal region, into the plurality of MSM actuator elements withoutprevious metrological determining of a crystal orientation in thesolidified MSM alloying material.
 29. The method according to claim 13,wherein the alloying material is NiMnGa.